Propulsion for the rocket was planned to be provided by four Merlin 1Drocket engines, engines that were also to be used in the Falcon 9 v1.1 beginning in 2013, and also on the Falcon Heavy in 2014. First flight for the air-launched Falcon 9 Air rocket was notionally planned for 2016.

Development ceased in 4Q2012 as SpaceX and Stratolaunch "amicably agreed to end [their] contractual relationship because the [Stratolaunch] launch vehicle design [had] departed significantly from the Falcon derivative vehicle envisioned by SpaceX and does not fit well with [SpaceX's] long-term strategic business model."[1]

In December 2011 Stratolaunch Systems announced that it would contract with SpaceX to develop an air-launched, multiple-stage launch vehicle, as a derivative of Falcon 9 technology, called the Falcon 9 Air,[2] as part of the Stratolaunch project.[3] As initially conceived with the SpaceX Falcon 9 Air (F9A) launch vehicle, Stratolaunch was to initially place satellites of up to 6,100 kg (13,400 pounds) into low-earth orbit; and once established as a reliable system, announced that it would explore a human-rated version.[4] The system can take-off from airfields with a minimum 3,700 m (12,100 feet) length, and the F9A carrier aircraft was proposed to travel to a launch point up to 2,200 km (1,200 nautical miles) away from the airfield and fly at a launch altitude of 9,100 m (30,000 feet).[3]

A month after the initial announcement, Stratolaunch confirmed that the first stage of the F9A launch-vehicle would have only four engines, not the five that were shown in the mission video in December, and that they would be SpaceX Merlin 1D engines.[5]

As initially announced, Stratolaunch Systems was a collaborative project that included subcontractors SpaceX, Scaled Composites, and Dynetics, with funding provided by Microsoft co-founder Paul G. Allen's Vulcan investment and project management company.[6] Stratolaunch set out to build a mobile launch system with three primary components: a carrier aircraft (aircraft concept was designed by Burt Rutan but the aircraft will be designed and built by Scaled Composites); a multi-stage launch vehicle to be developed and built by SpaceX; and a mating and integration system — allowing the carrier aircraft to safely carry and release the booster — to be built by Dynetics, a Huntsville, Alabama-based engineering company.[4] The whole system will be the largest aircraft ever built; with the first test flight of the carrier aircraft originally expected in 2015 from Scaled Composites' facilities in Mojave, California,[4] while the first test launch of the rocket was not expected before 2016 at the time of the project getting underway.[7]

As the Stratolaunch development program progressed, it became clear that Stratolaunch and the system integrator, Dynetics, wanted modifications to the SpaceX basic launch vehicle design that SpaceX felt were not strategic to the direction they were growing the company. These included requested modifications to the launch vehicle to add chines.[1]

On 27 November 2012 Stratolaunch announced that they would partner with Orbital Sciences Corporation—initially on an air-launched vehicle study contract—instead of SpaceX, effectively ending development of the Falcon 9 Air.[1]